Editorial Feature

Paper Thin Ferroelectric Nanogenerator For Next Generation Audio Technology

While most of the ultrathin materials that are produced in the electronics industry focus on enhancing the use of sight and touch, a team of Michigan State engineering researchers was more interested in creating a device that considered all human senses.

In December 2016, the team of researchers developed a unique biocompatible and flexible film-like device known as a ferroelectric nanogenerator (FENG), whose active ingredient is polypropylene ferroelectric (PPFE).

In their initial presentation of the FENG, the team of researchers utilized this material to successfully operate a liquid-crystal display (LCD) touch screen, 20 commercial green and blue light-emitting diodes (LEDs), and a self-powered keyboard.

The most recent breakthrough by these researchers further expands the potential of the FENG to capture sound vibrations as a source of mechanical energy, and then convert this to electrical energy.

With a thickness less than 100 mm, the FENG is comprised of polypropylene film, containing tiny foreign silicate particles that measure between 0.1-10 mm. When this film is stretched into perpendicular directions, an induced stress is applied to the silicate particles, leading to micro-crack formation that will create voids within the material.

During this fabrication process, high-pressure nitrogen and carbon dioxide (CO2) gas is diffused into the polypropylene film, which maintains the equilibrium between the internal and external pressures.

Upon reducing the external pressure, these voids dramatically swell, which can then be stabilized and stiffened at room temperature by thermal treatments. By applying a large electric field to this treated polypropylene film, Paschen breakdown occurs within the voids of the material that leads to a microplasma discharge to take place. Two thin layers of 500 nm silver are then deposited onto either side of the polypropylene film by sputtering.

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As a result of this makeup, the FENG is capable of converting mechanical energy into electrical energy, and vice versa. Within the film, as a result of the microplasma discharges, opposite charges accumulate in both the upper and lower surfaces of the voids, which form numerous large dipoles.

Either side of the sputtered silver will then attain this opposite polarity. Therefore, when an external force of mechanical energy, such as that caused by the pressure of touch, the previous arrangement of the dipoles will be reversed, leading to the flow of electrons in one direction.

When this pressure is released, the dipoles will restore their original sizes and allow for an opposite movement of the flow of electrons to occur. This process describes how the FENG converts mechanical energy into electrical energy to power a variety of devices.

Similarly, when electrical energy is supplied to the FENG, the giant dipoles will further expand in the event of a positive applied potential, and likewise shrink when a negative potential is applied. What is particularly impressive about the FENG is that each time the device is folded, it voltage and current signal production doubles, which presents a promising opportunity for much smaller devices to be operated by repeatedly folding this material.

Microphones, which are electroacoustic transducers, are capable of transforming acoustic vibrations into electrical signals, the FENG microphone exhibits a reversible electromechanical interaction that allows for its successful operation as both a microphone and a loudspeaker.

When an external sound pressure is applied, such as that which is caused by music or surrounding voices, it creates vibrations along the FENG surface, similar to that which is seen when touch is applied to the FENG. As a dual-functional device, the FENG-based microphone/loudspeaker are the same structure that employs opposite and reversible electromechanical interaction effects.

To further investigate this dual-functionality, the Michigan State researchers embedded the FENG fabric into their school flag, in which music was supplied to the flag by an amplifier connected to an iPad.

As a result of the inherent paper-like flexibility of the FENG, the flag could be waved, folded or rolled, without causing any type of physical or cosmetic difference to the FENG-based loudspeaker.

The FENG-based flag was discovered to successfully play music from both sides of its surface, while simultaneously exhibiting a wide audio frequency range.

The Michigan State team of researchers is hopeful that the previous successful incorporation of the FENG into such a diverse group of devices will be extended into potential future applications such as an audible newspaper, noise-cancelling sheeting, foldable loudspeakers, voice-activated security patches and much more.

Sources:

“Flexible and biocompatible polypropylene ferroelectric nanogenerator (FENG): On the path toward wearable devices powered by human motion.” W. Li, D. Torres, et al. Nano Energy. (2016). DOI: 10.1016/j.nanoen.2016.10.007.

Image Credit: Shutterstock.com/Physicx

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